1. Field of the Invention
This invention relates to the hatching of eggs of avian species, especially of poultry birds, and to the hatchlings thus obtained, adult birds grown from the hatchlings and subsequent generations of birds bred from said adult birds.
2. Description of Related Art
Sex determination in avian species, including domestic poultry (chickens, turkeys, pheasants etc) is determined by genetic mechanisms. Female birds are heterogametic, with ZW chromosomes, whereas males are homogametic, with ZZ chromosomes (this is the reverse of the situation in mammals, where males (XY) are the heterogametic sex and females (XX) are the homogametic sex. It is assumed that there are genes on either the Z or the W chromosome, responsible for sex determination in birds. However, it is unclear whether the genes which regulate male or female development are present on the Z chromosome, the W chromosome or on some combination of these (e.g. an activator on the Z chromosome and an inhibitor on the W chromosome). What is clear however, is that in birds, as in mammals, sex is apparently determined at the time of fertilisation (Sittman 1984, and references therein).
By contrast, in some reptiles, the temperature of egg incubation determines the sex of the offspring; there being no heteromorphic sex chromosomes in either males or females (see Deeming and Ferguson 1988 for a review),
It has therefore been supposed that there are two fundamental types of sexual determination in vertebrates: genetic (or chromosomal) sex determination as in birds and mammals, whereby the sex of the offspring is determined at the time of fertilisation and environmental (or temperature) determination in reptiles where sex is determined after fertilisation, as a result of interaction with some environmental agent.
There have been few previous investigations of the effects of temperature on sex ratio in birds, probably because deviations from the normal temperature in birds are usually lethal to the embryo (Bennett and Dawson 1979, Tazawa and Rahn 1986). In the domestic chicken, deviations from the preferred incubation temperature of 37.5.degree. to 38.degree. C. during incubation causes significant embryonic mortality. This situation in birds if very different from that In reptiles whose embryos are tolerant to a fairly wide range (often 5.degree. C. or more) of incubation temperatures without any significant effect on embryonic mortality.
However, Shubina et al. (1972) have reported that a change in incubation temperature of chicken eggs can alter the sex ratio between male and female embryos. The paper begins by referring to an earlier paper which is said to have shown that a brief increase in incubation temperature in the period preceding or concurrent with gonadal differentiation of chicken embryos leads to the formation of primarily female individuals. There follows a description of experiments apparently showing that reducing the temperature of incubation after 72 hours from 37.5.degree. C. to 22.degree. C. for 7 or 8 hours biases the sex ratio towards males, namely to a 1.5:1 or 1.43:1, respectively, ratio of males to females. These figures are not encouraging because mortality of the embryos was 9.1 and 10.5% respectively, compared with 6.1% in the control group: see Table 1. However, all figures in Table 1 must be treated with caution, since it is unclear how to reconcile the percentage mortalities with the total number of embryos examined and the total number of eggs used. Also, one set of experiments seems to have been omitted from the results shown in Table 1. Thirdly, the paper reports in Table 2 a previous experiment in which the incubation temperature is increased to 41.degree. C., yet the mortalities reported are only 7-12% and the bias is in favour of males.